High contrast silver halide emulsions are widely used in photographic materials and are of particular benefit in the field of graphic arts and in the manufacture of printed circuit boards. In such fields of technology, a higher contrast film can provide improved definition and clarity.
Films used in graphic arts commonly utilise emulsions comprising a mixture of silver chloride and silver bromide and are typically doped with a low level of rhodium (III) complexes, which enhances contrast by setting an exposure threshold, by permanently trapping photoelectrons produced by low levels of light exposure, above which a developable surface latent image may be formed.
Alternative dopants to the rhodium (III) complexes normally used are disclosed in U.S. Pat. No. 4,933,272, which is concerned with the internal incorporation of doping agents into silver halide grain lattices, including various iron, ruthenium, osmium, rhenium and iridium dopants.
WO-A-92/16876 discloses a method of increasing the contrast of photographic materials at low exposures, especially in photographic papers, by incorporating a dopant combination of an iridium dopant and among others an osmium dopant, including osmium nitrosyl pentachloride dopants.
GB-A-2282232 discloses a method of increasing the raw stock stability of red-sensitised photographic silver halide materials suitable for use, for example, in graphic arts films. The method involved incorporating in the silver halide emulsions red sensitising dyes of the trinuclear merocyanine type, the higher the concentration of which was incorporated the better the stability. The emulsions were also doped, as usual, preferably with rhodium, but optionally with ruthenium, osmium, iridium or other group VIII metals.
One problem that arises in photographic materials that contain high contrast emulsions, particularly those employing red sensitising dyes, is that over a period of time the traps provided by the rhodium (III) dopant materials can slowly become filled as a result of electrons from spurious sources and as a consequence, the speed of the emulsion tends to increase and the contrast tends to decrease. This problem is of particular relevance to graphic arts films where high contrast emulsions are utilised that contain relatively high concentrations of red sensitising dyes (because most of the laser diodes in use in the field are red light sources), which in the absence of light slowly inject electrons into the emulsion grains. Accordingly, nominally identical films of different age may have speed variability and contrast may decrease and image quality be reduced after prolonged storage.